Method and device for operating a pressure reservoir, in particular for common rail injection systems in automobile engineering

ABSTRACT

A method and to a device for operating a pressure reservoir, where during a compression phase in a pump chamber, a pump periodically increases the pressure of a fluid located therein, and by means of a discharge valve controlled by differential pressure fluid under high pressure is allowed to be introduced from the pump chamber into the pressure reservoir. During a decompression phase following a compression phase, fluid from a fluid reservoir is introduced into the pump chamber by means of a controllable intake valve. In order to be able also to operate the pressure reservoir without a high pressure measurement directly in the pressure reservoir, the fluid pressure in the pressure reservoir is ascertained by means of a pressure determination in the pump chamber. The pressure determination takes place indirectly, monitoring of the intake valve in the decompression phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of PCT/EP2015/054658,filed Mar. 3, 2015, which claims priority to German Application No. 102014 206 442.2 filed Apr. 3, 2014. The disclosures of the aboveapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to a method and device for operating apressure reservoir, in particular for common rail injection systems inautomobile engineering. The invention lies in the field of electricalengineering and mechanical engineering, and is concerned with devicesand methods for operating a pressure accumulator, in particular forcommon-rail systems in automotive engineering. More specifically, theinvention is concerned with the need for maintaining a controllablefluid pressure in a pressure accumulator of said type, in particularalso when fluid intentionally or unintentionally escapes from thepressure accumulator and new fluid must be introduced at high pressure.

BACKGROUND OF THE INVENTION

Normally, in the case of high-pressure injection systems in automotiveengineering, the fuel pressure in the pressure accumulator is regulatedto a setpoint pressure. The regulating system commonly comprises ahigh-pressure sensor in the high-pressure system, which high-pressuresystem detects the actual pressure and transmits this to the regulationsystem. If a high-pressure sensor of said type is defective, theregulation no longer functions, resulting in an overpressure or anunderpressure in the system. Owing to modern high-pressure systems withlittle leakage, it is possible, owing to the extensive optimization, forincorrect pressure levels to arise particularly easily in the event offailures. There is then a resulting risk in particular if nopressure-limiting valve is provided in the system. The use ofpressure-limiting valves however entails investment costs, which canpossibly be avoided if it is possible to replace a failing high-pressuresensor in an emergency situation.

Particularly high demands are placed on the function of the regulationsystem in the case of modern systems, too, in particular ifcorresponding pressure dissipation valves are not provided as analogvalves but are actively integrated as digital pressure dissipationvalves into the regulation.

Until now, to intercept the failure of a high-pressure valve, onlyemergency operation by way of a pilot control system is known, in thecase of which a possibly occurring overpressure is dissipated by way ofa system leakage by way of an analog pressure regulation valve. In thiscase, too, it can be attempted, without a pressure-limiting valve, tonevertheless avoid system overpressure.

In the case of pressure accumulators, in particular in the automotivesector in fuel injection systems, regulating systems are conventional inwhich a high-pressure pump delivers fuel into the pressure accumulatorand is actuated by way of a control variable. Normally, the deliveredfuel quantity is controlled by way of a metering valve in the feed lineto the high-pressure pump.

Injection systems of said type are known for example from DE 101 11 293A1 and from DE 10 2007 059 116 A1. Normally, such regulating systems arealso combined with pressure dissipation valves which permit thedissipation of overpressure in the pressure accumulator, which is notpossible in the supply branch of the fuel. Such a system is known forexample from DE 101 08 202 A1.

SUMMARY OF THE INVENTION

In the context of pressure accumulator systems in general, against thebackground of the prior art, the present invention is based on theobject of providing a method and a device for operating a pressureaccumulator, which method and device combine high operationalreliability with the least possible outlay in terms of construction andprocess, and make it possible for the pressure in the pressureaccumulator to be kept in a target range as reliably as possible.

Accordingly, the present invention relates to a method for operating apressure accumulator, in which method a pump periodically, during acompression phase in a pump chamber, increases the pressure of a fluidsituated therein, and fluid at high pressure is admitted from the pumpchamber into the pressure accumulator by way of adifferential-pressure-controlled discharge valve, and in which method,during a decompression phase following a compression phase, fluid isadmitted from a fluid reservoir into the pump chamber by way of acontrollable intake valve. In this context, the object on which theinvention is based is achieved in that the fluid pressure in thepressure accumulator is determined by way of a pressure determination inthe pump chamber.

Normally, the setpoint variable that must be adhered to in the case of apressure accumulator of said type is the setpoint pressure of the fluidin the pressure accumulator. Here, the fluid may be both a liquid and agas. The pressure accumulator may for example be a pressure accumulatorfor the supply of water, or may also particularly advantageously be apressure accumulator of a common-rail system in the context of a fuelinjection system in automotive engineering.

Normally, for the regulation of the pressure in the pressureaccumulator, high-pressure sensors are provided which detect thepressure directly in the pressure accumulator itself, and permitregulation to a setpoint variable. According to the present invention,however, the pressure in the pressure accumulator is determinedindirectly by way of a pressure determination in the pump chamber.

The pressure in the pump chamber is normally subject to relativelylarge, periodic, precisely defined pressure fluctuations, because it isnormally the case that the fluid to be delivered is initially introducedinto the pump chamber at a low pressure level and, there, is subjectedto a compression or other measures for the purposes of increasingpressure. Only when the pressure in the pump chamber has reached thesetpoint pressure in the pressure accumulator is fluid admitted from thepump chamber into the pressure accumulator. Thereafter, the connectionbetween the pump chamber and the pressure accumulator is shut off, andthe pump runs through a further cycle, in which fluid is admitted at arelatively low pressure level into the pump chamber.

Normally, during the discharging of the fluid from the pump chamber intothe pressure accumulator, fluid is delivered until the pressure in thepump chamber falls below the pressure in the pressure accumulator. Ifthe valve (discharge valve) that connects the pump chamber to thepressure accumulator is thereupon closed, it is possible, from the fluidpressure that remains in the pump chamber, to infer the pressureprevailing at that time in the pressure accumulator.

If, subsequently, thermodynamically replicable measures are implementedin the pump chamber, for example a decompression by reversal of a pumppiston, it is also possible, from a lowered pressure in the pumpchamber, to later also calculate the maximum pressure, if thecorresponding measures, for example the distance covered by the pumppiston and thus the volume expansion in the pump chamber, are known. Itis thus possible, by way of a pressure measurement in the pump chamber,to infer the pressure in the fluid chamber without a functioningpressure sensor having to be provided directly in the pressureaccumulator. This may be used for example if a pressure sensor is notprovided in the pressure accumulator at all, or if a pressure sensor inthe pressure accumulator has failed or is to be tested.

In this regard, an advantageous refinement of the invention may providethat the pressure in the pump chamber is measured at a time between theclosure of the discharge valve and the subsequent admission of fluidinto the pump chamber. Thus, from the changed pressure level in the pumpchamber after the discharge of the fluid into the pressure accumulator,for example as a result of a cyclic decompression phase in the case of apiston pump, the pressure in the pump chamber is determined, and thepressure at the time of the closure of the discharge valve iscalculated.

A further advantageous refinement of the invention provides that thepressure in the pump chamber is determined at the time of the opening ofthe intake valve, in particular by way of the determination of theposition of the pump piston at said time.

During the opening of the intake valve, it is normally the case that thepressure difference of the pressures in the pump chamber and in thefluid reservoir outside the pump chamber has to be overcome by a valve.An intake valve of said type may for example be in the form of adifferential pressure valve, which for example opens as soon as thepressure levels on both sides of the valve correspond or differ by adefined value. Such a valve may also exhibit a certain preload, forexample by way of a spring which is preloaded in an opening or closingdirection, such that a certain pressure difference between the twochambers must exist in order for the intake valve to open. Furthermore,it is also possible for the force that must be applied by a valveplunger in order to open the valve to be measured and taken intoconsideration. In each of these cases, it is however possible, if thepressure in the fluid reservoir is known, to infer the pressure in thepump chamber from this at the time of the opening of the intake valve.If, furthermore, the time of the opening of the intake valve in theoperating cycle of the pump is known, it is possible from this to inferthe position of a pump piston at the time of the opening of the intakevalve, and thus a compression ratio or a pressure change since the timeof maximum compression/closure of the discharge valve. In this way, itis possible to infer the maximum pressure attained in the pump chamberat the start of the decompression process, said pressure normallycorresponding to the pressure in the pressure accumulator, because, inthe range of the maximum pressure in the pump chamber, said pump chamberis connected to the pressure accumulator by virtue of the correspondingdischarge valve being opened.

If the intake valve is electronically actuated, it is possible, from theforce to be applied in order to open the valve or, if the valve is heldopen when deenergized, from the time of the opening in the pump cycle,to infer the differential pressure, and thus, with knowledge of thepressure in the fluid reservoir, to infer the present pressure in thepump chamber. In this case, too, it is possible, if the time of theopening of the intake valve is known, to calculate the pressure in thepump chamber before the start of the decompression.

An advantageous refinement of the invention therefore provides that,from the time of the opening of the intake valve, in particular from thetime difference between the opening time of the intake valve and thetime of the maximum compression of the pump or of the closing of thedischarge valve, the pressure in the pump chamber at the closing time ofthe discharge valve in the preceding compression phase is determined.

A further advantageous refinement of the invention may provide that theposition of a driveable pump piston, which delimits the pump chamber, atthe opening time of the intake valve is determined, in particular takinginto consideration the pump speed. From the pump speed, it is possible,for example by mathematical determination or else by determination in areference list in an evaluation device, to detect what position a pumppiston of the pump is situated in at a particular time at which apressure measurement in the pump chamber is possible, for example at thetime of the opening of the intake valve. The detection of the pump speedtogether with the detection of the time of the pressure determinationtherefore permits a reliable determination of the pressure in the pumpchamber in any other piston position of the pump piston, and thus alsoin the region of the transition from the compression phase into thedecompression phase, when the pressure in the pump chamber presentlyfalls below the pressure in the pressure accumulator.

In this respect, it is an advantageous aspect of the method according tothe invention that a compression ratio is determined from the positionof the pump piston at the opening time of the intake valve.

The invention may furthermore be advantageously refined such that theintake valve can be controlled electromagnetically by way of a currentflowing through a magnet coil and by way of an armature that can bedriven by the field of the magnet coil. The armature may for example beconnected to a plunger of the intake valve, on the end of which plungerthere is provided a valve closure element which can be closed against avalve opening. With a certain force which can be generated by themagnetic field of the magnet coil and which acts on the armature, it isthen possible for the valve to be opened, for example. From the currentthat must be applied to the magnet coil in order to generate a movementof the armature, it is possible to determine the force that must beovercome in the intake valve for opening purposes. This may be generatedfor example by a differential pressure which acts on both sides of thevalve, or by a pressing force which is exerted on the valve by apressing spring and which holds the valve in the closed position, forexample, until it is overcome by a differential pressure and/or by theforce of the armature. The valve may however also be held open whendeenergized, for example by way of an additional spring which acts inthe opening direction.

The invention may furthermore advantageously be refined such that thecurrent flowing through the magnet coil is monitored with regard to thecurrent intensity.

It may also be provided that a current signal, generated in the magnetcoil by an opening movement of the intake valve and of the armature, ofthe current flowing through the magnet coil is detected and is assignedan opening time of the intake valve. In this way, it is possible toprecisely determine the time at which the armature begins to move in thefield of the magnet coil, and thus the time at which the opening of theintake valve begins. At the same time, by way of the current intensityflowing through the magnet coil at said time, it is also possible todetermine the total force acting on the armature, and thus, withknowledge of the construction of the valve, for example also of providedpreload springs, it is also possible to determine the pressureprevailing in the pump chamber, or the differential pressure between thepressure in the pump chamber on one side of the valve and the pressurein the fluid reservoir on the other side of the valve. The time of saidpressure measurement is, as described above, determined by the currentsignal which is generated as a result of the start of the movement ofthe armature and thus as a result of the sudden change in the magneticcharacteristics of the system composed of magnet coil and armature. Aninductive action is realized, which can be identified for example as acurrent maximum or bend in the current profile curve. Such a signal canbe electronically discriminated, and thus the opening time of the intakevalve can be precisely determined.

The inductive action of the movement of the armature may also beutilized as an indicator for the opening movement if no opening force isgenerated by the magnet coil and the valve is held open by a spring whendeenergized. It is possible for a minimal current to be fed through themagnet coil, which generates practically no force on the armature butwhich permits easy identification of the induction on the current curve.

The invention relates not only to a method for operating a pressureaccumulator but also to a device for generating a fluid pressure in apressure accumulator, having a pump which has a pump chamber delimitedby a driveable pump piston, wherein the pump chamber is connectable atone side to the pressure accumulator by way of adifferential-pressure-controlled discharge valve and at the other sideto a fluid reservoir by way of a controllable intake valve, and havingan actuating device which controls the intake valve by way of anenergizable magnet coil and by way of an armature that can be driven bythe field of the magnet coil, and having a measurement device whichdetects the current flowing through the magnet coil with regard to thecurrent intensity and which monitors for a current signal generated by amovement of the armature in the field of the magnet coil.

A device of said type, by way of the measurement device which detectsthe current flowing through the magnet coil with regard to the currentintensity and which monitors for a current signal generated by amovement of the magnet armature, permits a precise detection of theopening time of the intake valve and thus, as described above, adetermination of the pressure in the pump chamber upon the closure ofthe discharge valve to the pressure accumulator. It is thus possible,even without a functioning pressure sensor in the pressure accumulator,for the pressure therein to be determined and monitored with acceptableaccuracy.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 schematically shows an overview of a device according to theinvention for generating a fluid pressure in a pressure accumulator;

FIG. 2 shows two typical current intensity profiles of the currentthrough the magnet coil by way of which the intake valve is controlled;

FIG. 3 shows the profile of the pump cycle, plotted versus the time,together with an illustration of the current profile in the magnet coil,which actuates the intake valve by way of an armature; and

FIG. 4 shows a method flow diagram for the determination of the pressurein the pressure accumulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

FIG. 1 schematically shows a pressure accumulator 1, which may be formedfor example by a common-rail pressure accumulator in a fuel injectionsystem of a vehicle. On the lower part of the pressure accumulator 1there are illustrated outlets 2, 3, where injection valves are commonlyarranged. For the sake of clarity, these have been omitted in thepresent drawing.

The device according to the invention is provided for providing fluid,in the present case that is to say a liquid in the form of fuel, in thepressure accumulator, or delivering said liquid into the pressureaccumulator, at high pressure, typically several hundred bar. For thispurpose, a pump chamber 4 is provided which is delimited in the fluidinlet region by a first wall 6, in the fluid outlet region by a secondwall 7, and additionally by a pump piston 5.

The first wall 6 has an opening 6 a through which fluid can flow from afluid reservoir 8 into the pump chamber 4. The opening 6 a can be closedby way of a first closure body 9, for example in the form of a cone, soas to form an intake valve such that no fluid can flow through theopening 6 a. For this purpose, by way of a first compression spring 11,the first closure body 9 is pushed away from the edge of the opening 6 awhich forms a valve seat, that is to say the spring 11 acts in anopening direction of the valve. The spring 11 may, contrary to thesimplified illustration in FIG. 1, act on the plunger 13 outside thepump chamber, for example in the region of the magnet coil 15.

Since the pressure in the fluid reservoir 8 is normally low, inparticular lower than that in the pump chamber 4, for example is atatmospheric pressure, it is necessary, during a compression phase, forthe valve 6 a, 9, 11 which closes the first wall 6 to be actuated oractivated in order to be closed. For this purpose, the valve plunger 13is provided which can pull the closure body 9 against the opening 6 aand against the valve seat. The valve plunger 13 is connected to amagnet armature 14, which moves in the field of the magnet coil 15 andwhich can be driven by virtue of the magnet coil 15 being energized. Themagnet coil 15 can thus have a current applied to it such that the valve6 a, 9, 11 is closed. For this purpose, a force must be applied by wayof the magnet coil 15 and the armature 14 which is high enough that thespring force and possibly the differential pressure between the pumpchamber 4 and the fluid reservoir 8 are overcome. The plunger 13 may beseparate from the closure body 9 or may in particular be connectedintegrally to said closure body. The spring on the closure body 9 isillustrated merely symbolically, and may be connected to the plungeroutside the pump chamber, for example within the magnet coil.

The current through the magnet coil is provided by way of a currentsource 16 and is monitored by way of a current measurement unit 17. Fromthe current flowing through the magnet coil 15, it is possible todetermine the magnetic force acting on the plunger 13 and thus on theclosure body 9.

The pump piston 5 in the pump chamber 4, or more precisely on theboundary surface of the pump chamber 4, is driven in cyclic fashion byway of a drive connecting rod 18 and a drive arm 19 of a pump motor 20.The solid lines in FIG. 1 indicate the pump piston approximately at thepoint of maximum compression in the pump chamber 4, that is to say inthe furthest upwardly situated position in FIG. 1. From there, the pumppiston 5 is, with an increase in size of the pump chamber 4, that is tosay during a decompression process, pulled into the lower position shownby dashed lines, and is moved cyclically upward again from there inorder to run through a further compression.

The fuel flows into the pump chamber during the entire downward movement(starting from the opening of the intake valve) of the piston. The valve6 a, 9, 11 is open when deenergized, and fluid can flow from the fluidreservoir 8 into the pump chamber 4. At the same time, the valve whichconnects the pump chamber 4 to the pressure accumulator 1, and which isformed substantially by the opening 7 a, the second closure body 10 andthe second compression spring 12, is closed. The constantly highpressure in the pressure accumulator 1 pushes the closure body 10against the opening 7 a in the second wall 7 and thus prevents the fluidfrom flowing out of the pressure accumulator 1 into the pump chamber andvice versa.

During the course of the upward movement of the pump piston 5 in thepump chamber 4, the valve 6 a, 9, 13, 14 is closed by energization ofthe coil 15, and the pump chamber 4 is closed off on all sides for aperiod of time. The pressure can increase as far as an upper extremeposition of the piston 5, wherein, at a certain time, such a highpressure is reached in the pump chamber 4 that the closure body 10 ispushed away from the opening 7 a in the second wall 7 counter to theforce of the second compression spring 12, and the pressure accumulator1 is connected to the pump chamber 4. For this purpose, fluid can flowover from the pump chamber 4 into the pressure accumulator 1, and thus,in the case of a common-rail pressure accumulator, fuel can bereplenished. When the pressure has equalized between the pump chamber 4and the pressure accumulator 1, and when the pump piston 5 movesdownward, initiating a decompression process, the closure body 10 ispushed against the opening 7 a again, and the fluid that has beenintroduced into the pressure accumulator 1 remains there.

Normally, the current through the magnet coil 15 is controlled suchthat, as a result of the closing time of the valve 6 a, 9, 11, 13, thefluid v, 13 that has moved from the fluid accumulator 8 through thevalve into the pump chamber 4 during the suction phase assumes aprecisely defined volume in the pump chamber. Following the compression,during an equalization movement of the compressed medium from the pumpchamber 4 into the pressure accumulator 1, pressure equalization betweenthe 2 chambers is achieved. In the subsequent decompression phase(pressure accumulator 1 already closed by valve 7 a, 10, 12), the mediumthat has already been previously compressed must be decompressed to alower pressure in the fluid accumulator 8 in order to permit asubsequent drawing-in of new medium. Only then will the valve 6 a, 9, 13b be able to open. To make it possible for the valve movement to bedetected and evaluated during said opening process, it is normally thecase that a low current is passed through the magnet coil 15, which lowcurrent is not enough to cause an actuation of the valve. Said current,and the reaction of a movement of the magnet armature on the current,can be detected by measurement, and it is thus possible for the time ofthe valve opening to be inferred. Depending on what pressure, reached asa result of the compression phase, has to be decompressed, an earlier orlater valve opening is evident in the current profile. The time of thevalve opening may be set in relation to the cyclic movement of the pumppiston or of the pump motor. If the pressure in the pressure accumulator1 falls, it is tendentially necessary for more fluid to be replenished,and, during the subsequent decompression phase, the valve 6 a, 9, 11opens at an earlier time than in the presence of a relatively highpressure in the pressure chamber. The time of the valve opening thusmakes it possible to indirectly determine the pressure in the pressurechamber 1.

Normally, the pressure build-up and the replenishment of fluid in thepressure accumulator are subject to regulation, wherein the monitoredpressure in the pressure accumulator 1 serves as a setpoint variable.Said pressure is normally monitored by way of a high-pressure sensor 21in the pressure accumulator. If a high-pressure sensor 21 of said typefails, or if it is the intention for said high-pressure sensor to betemporarily not used, or if said high-pressure sensor is temporarily notusable, then it is possible by way of the method according to theinvention for the pressure in the pressure accumulator 1 to bedetermined by way of an indirect measurement of the pressure in the pumpchamber 4.

In FIG. 2, it is schematically illustrated that the current I throughthe magnet coil 15, measured by way of the current measurement unit 17,varies over time. In the upper curve 22, upon an actuation of the magnetcoil 15, an increase of the current intensity is illustrated in the timerange 23. After passing through a maximum, the current fallsasymptotically owing to the induction action, wherein the magnetic fieldaction in the coil remains constant. At the time t₂, the pressure in thepump chamber 4 has fallen to such an extent that the spring force actingon the plunger 13 can effect a movement of the closure body 9 counter tothe differential pressure. Thus, at the time t₂, the plunger 13 moves,and it is thus also the case that the magnet armature 14 moves in thefield of the magnet coil 15. This gives rise to an inductive reaction onthe current, which is manifest in a bend 24 in the current curve, andwhich can thus also be verified by monitoring of the current intensityin a monitoring device 36, which is also connected to the pump motor.Through a detection of a bend point of said type, it is thus possible toidentify the time t₂ at which the force acting on the closure body 9 inthe opening direction exceeds the closure force of the valve exerted bythe differential pressure.

In the lower region of FIG. 2, there is illustrated a further currentcurve 25 which shows a corresponding, slightly different current signalin the form of a low current maximum, on the basis of which it can beverified that, in this case, at the time t₁, the magnet armature 14together with the plunger 13 has begun its opening movement.

The amplitude of the movement of the pump piston 5 is illustratedschematically by the curve 26 in the lower region of FIG. 3. The upperarcs of the sinusoidal curve show the states in which the pump piston 5moves upward during the course of a reduction in size of the pumpchamber 4 in FIG. 1 and effects a compression. Thus, in the diagram, thecurve 26 begins in a phase of maximum compression. At the time t₃, thepiston 5 moves downward during the course of a decompression, and thepressure falls initially until the time t₄. At the time t₄, the pistonhas reached a position in which, in the illustrated example, thepressure in the pump chamber 4 has fallen to such an extent that thevalve 6 a, 9, 11 opens to the fluid reservoir 8. The intake time periodof the intake valve is denoted by 27 in the diagram of FIG. 3, andextends until t_(5′). In the time period 27, it is thus the case thatfluid can flow over from the fluid reservoir 8 into the pump chamber 4.

After the pump piston 5 has passed through its bottom dead center andhas begun an upward movement again, at the time t₅, the valve 6 a, 9, 13is closed and the pump chamber is closed on all sides, and thus thecompression phase is initiated. The curve 26 rises, and the pressure inthe pump chamber 4 is increased. When a maximum pressure is reached atthe time t₆, the valve 7 a, 10, 12 between the pump chamber 4 and thepressure accumulator 1 opens, and, over an opening time 28, fluid athigh pressure can flow from the pump chamber into the pressureaccumulator 1.

The upper region of the diagram of FIG. 3 illustrates a cyclic currentprofile which represents the current intensity through the magnet coil15. In the region of the decompression movement of the pump piston 5after the time t₃, the current through the magnet coil is increasedslightly for the purposes of better detecting a valve movement. At thetime t₄, the pressure prevailing in the fluid reservoir 8 approximatelycorresponds to the already-decompressed pressure in the pump chamber 4,and the valve 6 a, 9, 13 (intake valve) subsequently opens with magneticforce assistance. This is evident from the current rise 29 that isgenerated as a result of induction during a movement of the magnetarmature, which current rise can be used as a signal for acknowledgementof the valve opening. After the opening time 27 of the intake valve hasbeen run through, the current through the magnet coil 15 can be shutoff. At the time t₅, the magnetic valve has a so-called closing pulse 50applied to it, which closes the valve 6 a, 9, 13 (intake valve) and thusinitiates the compression phase. In the diagram, the opening process isshown for a second time in the region of the curve 30, with thecorresponding current signal 31, at the time t₈.

From the detected valve opening times t₄, t₈ in the cycle of the pumpmovement 26 relating to the respectively preceding TDC (top dead center)of the pump, it is possible in each case to determine the time at whichthe pressure in the pump chamber has been depleted again after thepreceding compression phase. The pressure at the end of the compressionphase can be determined by way of a previously known correlation, whichis for example stored in a memory device, between valve opening time andpressure. The compression ratio in the pump chamber between the positionof the pump piston reached at the time t₄, t₈ and the maximally advancedposition of the pump piston, at which maximum compression is attained,is also known. It is thus possible to infer the pressure in the regionof maximum compression, which, owing to the opening of the valve 7 a,10, 12 in said time range, corresponds precisely to the pressure in thepressure accumulator 1. This may be realized in each case by calculationof the compression ratio; it is however also possible to realize acorrelation list of times t₄, t₈ at which an opening of the intake valvebegins and corresponding maximum pressures in the pump chamber obtainedby way of a calibration measurement.

FIG. 4 schematically shows the sequence of the method according to theinvention in a flow diagram, wherein a first step 32 indicates theidentification of a current signal 29, 31 including the identificationof the time of the current signal. In the second step 33, said time isset in relation to the profile of the pump piston movement, such that,from the known time of the current signal, it is possible to calculatethe position at which the valve 6 a, 9, 13 (intake valve) opens. Fromthe known time of the TDC (top dead center) of the pump and that of thevalve opening, determined by way of the measurable current rise (e.g. 29and 30), it is possible to infer the duration of the decompressionphase, and thus the pressure that previously prevailed in the pumpchamber 4 and in the pressure accumulator 1 that communicates therewith.From this, by way of the known pump parameters, in particular thedistance covered by the pump piston to the maximum position, or by wayof the known volume ratios in the position in which the pump piston issituated when the intake valve opens, on the one hand, and at the timeof the maximum compression, on the other hand, it is possible for theratio of the pressures at the time of the opening of the intake valve,on the one hand, and at the time of the closure of the discharge valve 7a, 10, 12, on the other hand, and thus the pressure in the pump chamberand in the pressure accumulator upon the closure of the discharge valve,to be calculated. This is performed in a fourth calculation step 35.

The method composed of the steps 32 to 35 may for example be performedimmediately as soon as it is detected that a pressure sensor in thepressure accumulator is defective. Furthermore, the method with thesteps 32 to 35 may, for the purposes of calibrating the method accordingto the invention, be performed in parallel with a pressure measurementby way of a high-pressure sensor in the pressure accumulator.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

What is claimed is:
 1. A method for operating a pressure accumulator, inwhich method a pump periodically, during a compression phase in a pumpchamber, increases the pressure of a fluid situated therein, and fluidat high pressure is admitted from the pump chamber into the pressureaccumulator by way of a differential-pressure-controlled dischargevalve, and in which method, during a decompression phase following acompression phase, fluid is admitted from a fluid reservoir into thepump chamber by way of a controllable intake valve, comprising the stepsof: determining the fluid pressure in the pressure accumulator by way ofa pressure determination in the pump chamber.
 2. The method as claimedin claim 1, further comprising the steps of measuring the pressure inthe pump chamber at a time between the closure of the discharge valveand the subsequent admission of fluid into the pump chamber.
 3. Themethod as claimed in claim 2, further comprising the steps ofdetermining the pressure in the pump chamber at the time of the openingof the intake valve, in particular by way of the determination of theposition of the pump piston at said time.
 4. The method as claimed inone of claim 3, further comprising the steps of determining the pressurein the pump chamber at the closing time of the discharge valve in thepreceding compression phase from the time of the opening of the intakevalve, in particular from the time difference between the opening timeof the intake valve and the time of the maximum compression of the pump.5. The method as claimed in claim 4, further comprising the steps ofdetermining the position of a driveable pump piston, which delimits thepump chamber, at the opening time of the intake valve, in particulartaking into consideration the pump speed.
 6. The method as claimed inclaim 5, further comprising the steps of determining a compression ratiofrom the position of the pump piston at the opening time of the intakevalve.
 7. The method as claimed in one of claim 6, further comprisingthe steps of controlling the intake valve electromagnetically by way ofa current flowing through a magnet coil and by way of an armature thatis driven by the field of the magnet coil.
 8. The method as claimed inclaim 7, further comprising the steps of monitoring the current flowingthrough the magnet coil is with regard to the current intensity.
 9. Themethod as claimed in claim 8, further comprising the steps of: detectinga current signal generated in the magnet coil by an opening movement ofthe intake valve and of the armature; and assigning an opening time ofthe intake valve.
 10. The method as claimed in one of claim 9, furthercomprising the steps of pushing the intake valve into the open positionwith a defined force by way of a preload spring.
 11. A device forgenerating a fluid pressure in a pressure accumulator, comprising: apump which has a pump chamber delimited by a driveable pump piston, thepump chamber being connectable at one side to the pressure accumulatorby way of a differential-pressure-controlled discharge valve and at theother side to a fluid reservoir by way of a controllable intake valve;an actuating device which controls the intake valve by way of anenergizable magnet coil and by way of an armature that can be driven bythe field of the magnet coil; and a measurement device which detects thecurrent flowing through the magnet coil with regard to the currentintensity and which monitors for a current signal generated by amovement of the armature in the field of the magnet coil.